Resistance material



Nov. 7, 1950 E. A. EVANS ETAL 2,529,144

RESISTANCE MATERIAL Filed March 29, 1945 3 Sheets-Sheet 1 Irwverwtors: Edmuhcl AEvans, (Barnett HP rter,

Their Attorney.

mvw ----w-- Nov. 7, 1950 E. A. EVANS ETA]. 2,529,144

RESISTANCE MATERIAL Filed March 29, 1945 3 Sheets-$heet 2 pulses ollow Current I? N N 2 4 Tale in "/0 of Bond Ratio of Follow Curreht after 5 im aFterZ impulses of L5 ka(|O-20ps wave) Q Fig.6.

Crest Amperes N b 6) O o O 2 4 s 8 IO Talc 1n7o of Bond Ivwvrwtors: Edmurwd A. Evans, Garne-tt HP flier, 04*B|.2I620 242s 525s 4 o4 4 y Hours Their Attorney.

Patented Nov. 7, 1950 RESISTANCE MATERIAL Edmund A. Evans and Garnett H. Porter, Pittsfield, Mass., assignors to General Electric Company, a corporation of New York Application March 29, 1945, Serial No. 585,474

13 Claims.

j above has been used extensively in electric dis- 1 charge devices such as lightning arresters. Y such material may be purchased on the market One under the name Thyrite. Many lightning arresters comprise a gap structure arranged in series with a resistance material of the above mentioned type. In the event of a surge voltage, such as may be caused by lightning, the gap structure breaks down completing a low resistance circuit through the resistance material to ground.

The resistance material is often referred to as the valve element. With this arrangement a low resistance path is provided between line and ground whereby lightning or switching surges can pass harmlessly to ground without damaging electrical equipment connected to the line. The valve element through its variable resistance characteristic permits the passage of the surge to ground without a large increase in voltage across it and yet it increases in resistance at line voltage sufiiciently to limit the power follow current due to line voltage to a value which the gap structure can interrupt. Since the higher passing through the resistance material the lower the resistance, no high potential will exist across the resistance material to injure associated apparatus.

It is obvious that since the entire surge passes through the resistance material, or valve element, and because the gap structure cannot interrupt the power follow current unless the power follow current is limited by the series valve element, this valve element must be able to withstand without puncture or flashover, or substantial change in electrical characteristics the entire discharge passed by the arrester due to lightning or switching surges. In View of the magnitude and duration of such discharges it is very dimcult to produce valve elements capable of withstanding the very severe discharges which can occur in nature.

It is an object of our invention to provide a new resistance material for use as the valve element in discharge devices with a much higher discharge capacity for withstanding long duration lightning and switching surges, such as those lasting for one millisecond or longer, than material used heretofore.

It is another object of our invention to provide a new and improved resistance material for use as the valve element in discharge devices which has a much greater permanence of protective characteristics when subjected to high current discharges than material used heretofore.

Still another object of our invention is to provide a new and improved resistance material for discharge devices having improved electrical characteristics, while at the same time being simple in construction and inexpensive to manufacture.

It is another object of our invention to provide a new and improved method of producing resistance material employed as the valve element in electric discharge devices.

Further objects and advantages of our inven tion will become apparent as the following description proceeds and the features of novelty which characterize our invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

For a better understanding of our invention, reference may be had to the accompanying drawings wherein Fig. 1 is an elevational view, partly in section, of a discharge device employing the resistance material of our invention; Fig. 2 is a perspective view of a disk of resistance material embodying our invention employed in the discharge device of Fig. 1; Fig. 3 is a view similar to Fig. 1 of another discharge device embodying a valve element employing the resistance material of our invention; Fig. 4 is a View similar to Fig. 2 of a disk of resistance material employed in the discharge device of Fig. 3; Figs. 5, 6 and '7 are curve diagrams to aid in understanding our invention, and Figs. 8, 9, 10 and 11 illustrate certain characteristics of the resistance material of our invention before and after conditioning.

The discharge device illustrated in Fig. 1 is in many respects similar to that disclosed in McEachron Patent 2,151,559, granted March 21, 1939, and assigned to the same assignee as the present application. Except for the specific resistance material employed in the discharge device, Fig. 1 is substantially identical with the disclosure in copending application, Serial No.

572,508, Zimmerman, filed January 12, 1945, now Patent No. 2,495,154, issued January 17, 1950, and assigned to the same assignee as the present application. This electric discharge device, generally indicated as a lightning arrester I in Fig. 1, comprises a housing 2 within which is mounted a sealed gap structure 3 comprising a plurality of series gaps 4. In series with the gap structure 3 is a valve unit generally indicated at 5 comprising a plurality of resistance plates or disks 6, arranged in a stack with adjacent faces in engagement, which adjacent faces Preferably are each provided with a coating of conducting material such as metal. Preferably these plates, or disks, comprise an outer annular member or collar 1 of insulating material such as a suitable ceramic containing a core of resistance material 8 constructed in accordance with our invention, as will be described hereinafter. The lightning arrester l is provided with a line terminal 9 and a ground terminal H].

In Figs. 3 and 4 we have illustrated a somewhat different form of lightning arrester which, however, employs a similar type of resistance plat or disk. The lightning arrester, or discharge device in Fig. 3 generally indicated at H, is substantially identical with the lightning arrester disclosed and claimed in United States Letters Patent 2,422,978, Olsen, granted June 24, 1947, and assigned to th same assignee as the present application. As illustrated, discharge device or lightning arrester l I, comprises a sealed cylindrical casing [2 within which are mounted a plurality of disks l3 formed of the resistance material of our invention alternately arranged with a plurality of gap units I4. The alternately arranged gap units IA and resistance disks l3 are mounted within sealed casing l2 and electrically connected between a line terminal l5 and a ground terminal [6. The resistance disk l3 as is best shown in Fig. 4, comprises an annular ring or collar I! formed like collar 1 described above, and a core [8 of resistance material constructed in accordance with our invention to be described hereinafter.

From the above description, it will become apparent that the disks 6 and I 3 are generally quite similar each comprising a collar of insulating material I and I1, respectively. The collars I and i1 prevent fiashover. The disks 6, as illustrated, are both larger in diameter and of greater thickness than the disks I3, but it should be understood that any desirable disk sizes may be employed.

The improved resistance material of our invention comprises silicon carbide 0r carborundum grains or particles held together by a, suitable binder or bonding material. In order that the resistance material employed in disks 6 and [3 has greater strength against long duration lightning and switching surges and greater permanence of protective characteristics when subjected to high current discharges than material used heretofore, we introduce a predetermined amount of talc into the bonding material and We furthermore condition the disks by the application of controlled impulses of predetermined magnitudes, durations and polarities, said introduction of talc and impulsing being both jointly and separately beneficial.

We have found that the addition of small quantities of talc to the bonding material or binder for the silicon carbide particles makes the bond more reactive and more glassy or fluid during vitrification which in turn results in the bonding material making more intimate contact with larger areas of the silicon carbide grains. On solidifying, the talc bearing bonds apparently grip the silicon carbide grains more tightly than bonds used heretofore and bring the grains into tighter contact and produce a greater number of contacts. This reduces disk resistance since a large part of the disk resistance is in the intergrain contacts. The lower resistance of these contacts reduces the voltage stress across them at any current over the range expected for switching surges or long duration lightning discharges and therefore they will carry a higher current before reaching failure voltage. It is also possible that the greater fluidity of the talcbearing bond results in its distribution over a larger proportion of the grain surface and therefore provides more contacts in which the insulation is very thin. This, of course, provides a construction in which the material is susceptible to greater improvement by electrical impulsing as will be brought out hereinafter. The bonding material is an insulator and moderate quantities between grain surfaces can electrically insulate the grains from each other. It should be understood that the above discussion setting forth the theory of why the presence of a predetermined amount of talc improves the resistance disk is given by way of explanation only and we do not wish to be limited to this theory in the event that it is incorrect and the advantageous features of our resistance material are accounted for in some different manner.

We have found that a very desirable resistance material can be constructed of a mixture com prising 75% silicon carbide particles and 25% bonding material. These silicon carbide particles or grains may have a grain size, such as will pass through a sieve mesh of the order of 20 to 200 depending on the particular application. A satisfactory bonding material might comprise a porcelain mixture which, in accordance with our invention, is mixed with a small percentage of talc, such as steatite, or hydrated magnesium silicate. We have found that 5% talc in bonding material is very satisfactory although it is possible to obtain the advantages of our invention with a percentage of talc within the range of 2 to 10% of the total bond. As a matter of fact, the percent of the bond which is talc is very important. If too much tale is used, say over 10%, the power follow current will be greater than the gap structure serially arranged with the disks can seal, and furthermore, the disks may be unsatisfactory mechanically. The influence of talc on the follow current is clearly shown by the curve of Fig. 5. If too little talc is used in the bond, say less than 2% of the bond,'the strength on long duration discharges which are discharges lastin one millisecond or longer often referred to as long duration strength, will be too small. The influence of talc on the long duration strength is clearly shown in Fig. 6 wherein the ordinates are plotted as the magnitude of the long duration impulse currents of particular wave form that the disks withstood Without puncture. In accordance with the above discussion a particular resistance material of our invention might comprise a mixture of 75% silicon carbide particles, 23.75% porcelain mixture and 1.25% talc. The porcelain mixture Will comprise clay, feldspar, flint and the like.

It is very essential that the very small percentage of talc is uniformly distributed throughout the batch of material. In the preferred method of constructing the resistance material of our invention the talc is first thoroughly mixed with the porcelain mix. After this the silicon carbide grains or particles are added to the bond and the batch is thoroughly mixed again. The batch of material comprising the mixture set forth above is then placed within a cylinder or dam which in turn is placed within a mold so as to define an annular space into which material suitable for an insulating collar is then placed. After suitable manipulation the dam is removed and pressure is applied to the core of the disk and its collar simultaneously. Successive applications and releases of pressure to permit the escape of trapped air is desirable in order to obtain a disk free from mechanical defects. Preferably pressures of 4,000 to 12,000 pounds per square inch are applied to the resistance material.

After the resistance material has dried, it is fired in accordance with a closely controlled temperature time cycle, such for example as is indicated by the curve of Fig. 7. For a particular construction we have found it to be desirable that the top temperature should be within the range of 1210 C. to 1240 C. Preferably, the top temperature employed in firing specific disks of resistance material made in accordance with our invention is 1225 C. It should be understood that for disks of difierent composition a different controlled temperature time cycle might be desirable.

As was mentioned above, certain improved characteristics of resistance material of the type referred to above may also be brought about by impulsing the fired disks in a predetermined manner. We do not wish to be bound by the following theory explaining how the improved characteristics are obtained, although it is believed to be a logical explanation. When the disks are subjected to impulses of the kind described hereinafter, it is believed that these impulses break down some of the insulating films of the bonding material which are present between some of the silicon carbide grains. As a consequence thereof more paths for carrying the discharge current are provided. It is believed that this may be more clearly understood by reference of Figs. 8, 9, l0, and 11. Fig. 8 illustrates a sectional view taken through a disk 6 before conditioning in accordance with our invention, while Fig. is an identical View of the same disk after conditioning. The dashed lines provided with arrowheads in Fig. 8 indicate the nonuniform current distribution through the disk by virtue of the fact that the portions of the disk such as I!) to 25 inclusive contain silicon carbide particles insulated from each other by bond films as is clearly apparent in the greatly magnified view of portion #9 il lustrated in Fig. 9. The silicon carbide grains are indicated at 25 while the bond insulating films are indicated by the dark spots designated as 21. Actually the bond is not all concentrated in the dark spots but is distributed throughout many of the spaces between grains. Also the silicon carbide grains are dark in color while the bond is white, but for the purpose of more clearly illustrating our invention the bond films which are punctured when the disk is conditioned are shown as dark spots.

When the disk is conditioned in accordance with our invention, as will be brought out in greater detail hereinafter, the insulating bond films 21 are punctured and the current paths through portion H! are clearly shown in Fig. 11. Fig. 10 shows disk 6 after conditioning and brings out the fact that now the current. is distributed in a much more nearly equal manner. Regions of overstress by virtue of current concentration are largely eliminated.

It is apparent that the passage of a discharge through the various chains of grain contacts will apply more voltage to the higher resistance contacts and heat these higher resistance contacts to a greater extent than the lower resistance contacts. Because the high resistance contacts get hotter than the low resistance contacts, the contacts are modified so as to become lower in resistance and this should result in making the resistance per contact more uniform. In other words, the voltage gradient through the disks will be more uniform after conditioning. Each portion of a path will therefore take a more equal portion of the voltage stress and the strength of the disk should be increased thereby. These theories were substantiated by connecting two disks of the same original composition and characteristics in parallel so that when an impulse was discharged through them each would have the same voltage across it. One of these disks was conditioned with three impulses of 5000 amperes having a wave shape which would reach crest current in 9 micro-seconds and reduce to half current in 16 micro-seconds. The other disk was unconditioned. The test showed that the conditioned disk carried twice the current which passed through the unconditioned disk. Furthermore, even though the unconditioned disk carried only half as much current and had released in it only half the energy, it failed while the conditioned disk did not. This test was repeated for the same conditioned disk paired successively with three unconditioned disks. In each case the unconditioned disk punctured and the conditioned disk did not. This complete set of tests was repeated twice more with new conditioned disks in each case. It is probable that the improvement in disk strength of the three conditioned disks was due to a larger area of the conditioned disks carrying the discharge as set forth above and, due to a more uniform gradient, distributing the voltage stress more evenly throughout the disk.

It will be obvious that the disks of resistance material may be conditioned by a wide variety of impulses. In accordance with our invention we condition disks with both negative and positive impulses because this makes the disks much stronger against positive and negative surges. If we should condition only with one polarity, say with positive polarity, the disk would then be much stronger against negative surges, but only moderately stronger against positive surges. Similarly, conditioning with only negative surges would make the disk much stronger against positive surges, but only moderately stronger against negative surges. Therefore, we condition with both polarities in order greatly to increase disk strength against surges of either polarity.

In accordance with our invention, we first condition a disk with two negative impulses to the side of the disk which will be toward the line and then follow with two positive impulses. This polarity sequence is used because it leaves the disk with a, little greater strength against negative surges than positive surges. This is preferable because lightning surges are usually negative, whereas ordinary switching surges are equally liable to be positive or negative. The disks 6 of Fig. 2 in accordance with our invention are conditioned with two negative impulses of 10,000 amperes followed by two positive impulses of 10,000 amperes. These impulses have a wave shape such that the crest current is reached in micro-seconds or in the range of 10 to micro-seconds and reduces to half current in 20 micro-seconds or in the range of 20 to micro-seconds. Preferably the disks 13 of Fig. 4 are conditioned with 5,000 ampere impulses of the same number, polarity, sequence, and duration as disks 6.

If currents of higher magnitude and equal or longer times than those mentioned above are used for conditioning, the power follow current will be unnecessarily increased and impose a more severe sealing duty on the gap structure. On the other hand, if currents of lower magnitudes and equal or shorter times are used, the long duration strengths of the valve element will be reduced. An impulse of slightly greater magnitude, but of shorter length may be very desirable. For example, an impulse reaching a crest magnitude of 13,000 to 15,000 amperes in 1 to 4 microseconds and reducing to half current in 3 to '7 microseconds produces a very strong disk. Theoretically such a short high current impulse should be desirable in that the voltage pulse required to break down the insulating films between particles of silicon carbide is applied for such a short time that there is less danger of damage to the contacts between the particles. Mechanically it is also desirable in that the impulse generator for producing such a wave is physically small. Another desirable conditioning wave would be a highly oscillatory wave having equal or nearly equal magnitudes for positive and negative current lobes since it would condition with both positive and negative discharges in one operation. It is difficult, however, to obtain such a wave because circuit limitations usually result in successive lobes having considerably smaller current magnitudes.

We have found that by the use of talc and by conditioning the valve disks constructed in accordance with our invention in the manner described above, greater permanence of protective characteristics will be provided. We have discovered that if the resistance material of the prior art is subjected to very high current discharges of the order of 100,000 amperes the voltage across the valve elements and therefore the voltage across the equipment which they are protectin will increase substantially at lower currents of the order of hundreds of amperes. In contrast to this, under similar conditions the voltage across the new valve element constructed in accordance with our invention remains substantially unchanged. It is very undesirable to have the resistance material employed as the valve element in a lighting arrester change greatly in protective characteristics due to being subjected to 'a high discharge current. With our invention not only is greater permanence of characteristic obtained, but in addition greater strength against long duration lightnin and switching surges is obtained.

The embodiments of the invention illustrated and described herein have been selected for the purpose of clearly setting forth the principles involved. It will be apparent, however, that the invention is susceptible of being modified to meet the different conditions encountered in its use and we therefore aim to cover by the appended claims all modifications within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A resistance material including a mass of silicon carbide particles and a binder mixture for holding adjacent particles in contact, said binder mixture comprising porcelain and 5% talc.

2. A resistance material including a mass of silicon carbide particles and a binder for holding adjacent particles in contact comprising a mixture of 75% silicon carbide particles, 23.75% porcelain mixture, and 1.25% talc.

3. The method of producing a resistance material for lightning arresters comprising the steps of mixing together 95% porcelain with 5% talc to produce a bond, mixing said bond with a mass of silicon carbide particles, pressing said mixture of bond and silicon carbide particles into disks by utilizing pressures between 4,000 to 12,000 pounds per square inch, drying said disks, heating said disks to a temperature between 12l0 and 1240 C., and finally subjecting said disks to a plurality of high current impulses each having a duration of the order of a small fraction of a second.

4. The method of conditioning a resistance material comprising silicon carbide grains bonded together with a suitable permanent binder containing 2-10% talc in order to increase the strength thereof against long duration surges of high current value and to obtain improved permanence of the initial characteristics thereof which comprises discharging through said material a plurality of high current impulses, each of which has a duration of the order of a small fraction of a second.

5. The method of conditioning a resistance material comprising silicon carbide grains bonded together with a suitable permanent binder containing 2-10% talc in order to obtain a high permanence of characteristics when subjected to high current discharges while in use and to obtain increased strength against long duration surges of high current value which comprises discharging through said material two successive current impulses of opposite polarity, each of which has a duration of theorder of a small fraction of a second.

6. The method of conditioning a resistance material comprising silicon carbide grains bonded together with a suitable permanent binder containing 2-10% talc in order to obtain improved permanence of the characteristics of said material and to increase the strength of said material against long duration surges of high current value which comprises discharging through said material two successive high current impulses of one polarity followed by two successive impulses of opposite polarity, each of said impulses having a wave shape such that the crest current is reached in the range of 10-20 microseconds and reduced to half current in the range of 20 to 40 micro-seconds.

7. The method of conditioning an improved resistance material in the form of a fired disk comprising silicon carbide grains bonded together with a suitable permanent binder containing 210% talc in order to greatly increase the ability of the disk to discharge lower current long duration impulses of the order of hundreds of amperes without puncture of the disk which comprises subjectin said disk to high current impulses of the order of thousands of amperes and each of which has a duration of the order of a small fraction of a second.

8. The method of conditioning a value element for a lightning arrester in the form of a disk of resistance material comprising silicon carbide grains bonded together with a suitable permanent binder in order to obtain greater strength against long duration lightning and switching surges than unconditioned disks, which comprises discharging two impulses of negative polarity through said disk from the line side thereof when employed in a lightning arrester followed by two impulses of positive polarity, each of said impulses having a duration of the order of a small fraction of a second.

9. The method of producing a resistance material for lightning arresters comprising the steps of mixing together a porcelain with from 2 to 10% tale to produce a bond, mixing said bond with a mass of silicon carbide particles, pressing said mixture of bond and silicon carbide particles into a disk by utilizing pressures between 4,000 to 12,000 pounds per square inch, drying said disks, heating said disks to a temperature of the order of 1225 C., and finally conditioning said disk by subjecting it to a high current impulse having a duration of a small number of micro seconds to provide an improved conducting path between the particles of silicon carbide through the bond.

10. The method of producing a resistance material for lightning arresters comprising the steps of mixing together a porcelain with from 2 to 10% talc to produce a bond, mixing said bond with a mass of silicon carbide particles, pressing said mixture of bond and silicon carbide particles into disks by utilizing pressures between 4,000 to 12,000 pounds per square inch, drying said disks, heating said disks and finally subjecting said disks to a plurality of high current impulses each of which has a duration of the order of a small fraction of a second to provide an improved conducting path through the bond between the particles of silicon carbide.

11. A resistance for use in a lightning arrester comprising a mixture of approximately 75% silicon carbide particles and approximately 25% binder material, the binder material comprising 90 to 98% porcelain and 2 to 10% talc.

12. The method of producing a resistance material for lightning arresters comprising the steps of mixing together a porcelain with from 2 to 10% talc to form a permanent bond, mixing approximately 25% of said bond with 75% of silicon carbide particles, pressing said mixture of bond and silicon carbide particles into a disk, heating said disk to the point of vitrification and finally conditioning said disk by subjecting it to a high current impulse having a duration of the order of a small fraction of a second to provide an improved conducting path between the particles of silicon carbide through the bond.

13. A resistance material including a mass of silicon carbide particles and a binder mixture for holding adjacent particles in contact, said binder mixture comprising porcelain and 2 to 10% talc.

EDMUND A. EVANS. GARNET'I H. PORTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number 

1. A RESISTEANCE MATERIAL INCLUDING A MASS OF SILICON CARBIDE PARTICLES AND A BINDER MIXTURE FOR HOLDING ADJACENT PARTICLES IN CONTAACT, SAID BINDER MIXTURE COMPRISING 95% PORCELAIN AND 5% TALC. 